There are gas burning dryers, but I've only ever personally seen or used electric ones (resistive heaters), it's there to vent the damp air outside. I'd personally like a heatpump dryer, as far as I know they don't need any venting.

The vent is for the moisture-laden warm air that results from forcing hot air into it. Possible than vented dryers are less common in your region if you are not familiar? They are an incredibly simple design, and thus very cheap to make, but are significantly less energy efficient than modern condenser or heat-pump dryers.